1. Field of the Invention
The present invention relates, in general, to a sensor for detecting the amount of partial discharge required to inspect insulated conditions of power devices and, more particularly, to a hybrid type sensor for detecting high frequency partial discharge, which is not influenced by power noise and surrounding noise, and is capable of safely detecting the amounts of high frequency partial discharge ranging from a few mV to several hundred mV at a high signal-to-noise ratio even though a high surge voltage is input to the sensor due to breakdown.
2. Description of the Related Art
Generally, an inspection for insulated conditions of power equipment is performed by analyzing partial discharge radiated from the power equipment. However, since the level of a partial discharge signal is as low as a few mV to several hundred mV, external noise may be easily input to the power equipment. Therefore, there have progressed researches for High Frequency Partial Discharge (HFPD), which improves a Signal-to-Noise (S/N) ratio by measuring partial discharge in a high frequency band in which external noise is not easily input.
A HFPD detecting scheme defined in IEC 270 is implemented so that a small amount of partial discharge current generated in an inspection object is guided to a coupling capacitor by using the coupling capacitor and the partial discharge current is measured at a terminal connected to a ground side. In this case, the capacitance of the coupling capacitor is prescribed to be two times or more the capacitance of the inspection object. However, in many cases, the capacitance of the inspection object is actually greater than that of the coupling capacitor. Therefore, the HFPD detecting scheme is problematic in that it is difficult to measure a partial discharge current due to the limitation of the capacitance of the coupling capacitor, and, additionally, a large-sized inspecting device and many investment costs are required.
Various sensors for measuring the HFPD, such as inductive, capacitive or resistive sensors, or sensors used for antennas, have been proposed according to their uses. The HFPD measuring sensors are constructed so that they take signals obtained from the sensors, signals picked up by the antennas for surrounding noise, or the propagating direction of the signals into consideration, analyze partial discharge patterns by a computer, and then extract only a partial discharge signal from the analyzed results.
There are two types of HFPD measuring sensors most generally used in the prior art: a Current Transformer (CT) type sensor and a shunt type sensor.
The CT type sensor, which is a non-contact type sensor, shows a high S/N ratio in a specific frequency band because it uses a resonance point fc, but oscillation occurs to decrease sensitivity.
The shunt type sensor has the most ideal precision, but it is disadvantageous in that, when breakdown occurs, a high surge voltage may be induced at an input terminal of the sensor to damage a test system and power noise may be input to the sensor. Further, theoretically, the shunt type sensor detects partial discharge with respect to the frequencies of all bands. However, actually, self-resonance occurs at higher frequencies due to stray capacitance or inductance caused by a lead wire, so that impedance is increased at higher frequencies. Therefore, it is difficult to measure partial discharge in high frequency bands. Moreover, as shunt resistance is increased, a power frequency is further increased, thus decreasing a S/N ratio. Therefore, in the case of the conventional shunt type sensor, there are required the selection of a suitable shunt resistance and measures for the input of power noise.
As described above, since the conventional high frequency partial discharge detecting sensors have many problems, it is difficult to apply the sensors to actual inspection fields.